Transformer with controlled leakage inductance

ABSTRACT

A multi-leg transformer includes a core having a plurality of center posts, a primary coil wound around at least one of the center posts, a secondary coil wound around at least one of the center posts and spaced apart from the primary coil, and at least one magnetic shunt material disposed in one or more selected areas between the primary coil and the secondary coil.

CROSS-REFERENCE TO RELATED APPLICATION

This patent application claims the priority benefit under 35 U.S.C. §119(e) of U.S. Provisional Application No. 63/344,819, filed on May 23,2022, the contents of which are herein incorporated by reference.

FIELD OF THE INVENTION

The disclosed concept relates generally to electrical components, andmore particularly, to transformers.

BACKGROUND OF THE INVENTION

Transformers face numerous design challenges. For example, in higherpower transformer applications cooling is a concern. As another example,in transformers used in LLC applications the size of the core gaps andfringing flux are concerns. Additionally, the leakage inductance oftransformers affects the suitability of transformers in certainapplications and is compensated for by use of an additional inductor.There remains room for improvement in transformers.

SUMMARY OF THE INVENTION

According to an aspect of the disclosed concept, a multi-leg transformercomprises: a core having a plurality of center posts; a primary coilwound around at least one of the center posts; a secondary coil woundaround at least one of the center posts and spaced apart from theprimary coil; and at least one magnetic shunt material disposed in oneor more selected areas between the primary coil and the secondary coil.

According to an aspect of the disclosed concept, a method of making amulti-leg transformer comprises: providing a core having one or morecore legs, a primary coil wound around at least one of the core legs,and a secondary coil wound around at least one of the core legs andspaced apart from the primary coil; determining a desired level ofleakage inductance of the multi-leg transformer; and tuning themulti-leg transformer to the desired level of leakage inductance byplacing at least one magnetic shunt material in one or more selectedareas between the primary coil and the secondary coil.

According to an aspect of the disclosed concept, a bobbin for amulti-leg transformer having a core having a plurality of center posts,a primary coil wound around at least one of the center posts, asecondary coil wound around at least one of the center posts and spacedapart from the primary coil, and at least one magnetic shunt materialdisposed in one or more selected areas between the primary coil and thesecondary coil comprises: a plurality of center portions correspondingto the plurality of center posts, each center portion structured to havea coil of the primary coil or secondary coil wound around it; at leastone insulating barrier disposed between two of the plurality of centerposts and structured to provide insulation between coils of the primarycoil or the secondary coil; and at least one capture feature structuredto assist the primary coil or secondary coil retain shape around thecenter posts.

BRIEF DESCRIPTION OF THE DRAWINGS

A full understanding of the disclosed concept can be gained from thefollowing description of the preferred embodiments when read inconjunction with the accompanying drawings in which:

FIG. 1A is an isometric view of a multi-leg transformer in accordancewith an example embodiment of the disclosed concept;

FIG. 1B is a top view of the multi-leg transformer of FIG. 1A;

FIG. 1C is a front view of the multi-leg transformer of FIG. 1A;

FIG. 1D is a side view of the multi-leg transformer of FIG. 1A;

FIG. 2A is a top view of a multi-leg transformer in accordance with anexample embodiment of the disclosed concept;

FIG. 2B is a cross-sectional view of the multi-leg transformer of FIG.2A;

FIG. 3 is an exploded front view of a multi-leg transformer inaccordance with an example embodiment of the disclosed concept;

FIG. 4 is an exploded isometric view of a multi-leg transformer inaccordance with an example embodiment of the disclosed concept;

FIG. 5A is an isometric view of a lower core of a multi-leg transformerin accordance with an example embodiment of the disclosed concept;

FIG. 5B is a top view of the lower core of FIG. 5A;

FIG. 5C is a front view of the lower core of FIG. 5A;

FIG. 5D is a side view of the lower core of FIG. 5A;

FIG. 6A is an isometric view of a bobbin of a multi-leg transformer inaccordance with an example embodiment of the disclosed concept;

FIG. 6B is a top view of the bobbin of FIG. 6A;

FIG. 6C is a front view of the bobbin of FIG. 6A;

FIG. 6D is a side view of the bobbin of FIG. 6A;

FIG. 7A is an isometric view of a winding of a multi-leg transformer inaccordance with an example embodiment of the disclosed concept;

FIG. 7B is a front view of the winding of FIG. 7A; and

FIG. 7C is a side view of the winding of FIG. 7A.

DETAILED DESCRIPTION OF THE INVENTION

Directional phrases used herein, such as, for example, left, right,front, back, top, bottom and derivatives thereof, relate to theorientation of the elements shown in the drawings and are not limitingupon the claims unless expressly recited therein.

As employed herein, the term “number” shall mean one or an integergreater than one (i.e., a plurality).

Various views of a multi-leg transformer 10 in accordance with anexample embodiment of the disclosed concept are shown in FIGS. 1A-D.FIGS. 2A-B show additional views, including a cross-sectional view ofthe multi-leg transformer 10, and FIGS. 3 and 4 show exploded views ofthe multi-leg transformer 10.

The multi-leg transformer 10 includes an upper core 20 and a lower core22, an upper bobbin portion 30 and a lower bobbin portion 32, a primarycoil 40 and a secondary coil 42, and shunt material 50. FIGS. 6A-D showadditional views of the lower core 22, FIGS. 6A-D show additional viewsof the upper bobbin portion 30, and FIGS. 7A-C show additional views ofthe primary coil 40. It will be appreciated that the upper core 20 andlower core 22 may be the same or similar, the upper bobbin portion 30and the lower bobbin portion 32 may be the same or similar, and theprimary coil 40 and secondary coil 42 may be the same or similar.

In an example embodiment of the multi-leg transformer 10, the upper andlower cores 20,22 form a multi-leg e-e or e-I core style structurehaving multiple center posts 24,25,26 and outer posts 27,28 (shown inFIGS. 5A-D). However, it will be appreciated that cores having anynumber of core or center posts greater than one may be employed. It willalso be appreciated that the center posts 24,25,26 may have any shape,for example and without limitation, rounds, square, oblong, rectangular,or other shapes without departing from the scope of the disclosedconcept. The primary coil 40 is wound around the center posts of theupper core 20 and the secondary coil 42 is wound around the center postsof the lower core 22, although it will be appreciated that the primaryand secondary coils 40,42 may be switched without departing from thescope of the disclosed concept. It will also be appreciated that theprimary and secondary coils 40,42 may be wound around any number of thecenter or outer posts without departing from the scope of the disclosedconcept. The upper and lower bobbin portions 30,32 are structured tofacilitate the winding and placing of the primary and secondary coils40,42 around their corresponding core posts.

In an example embodiment, the multi-leg transformer 10 is structuredsuch that the primary and secondary coils 40,42 are flat windings thatare spaced apart. However, it will be appreciated that the primary andsecondary coils 40,42 windings may be varied without departing from thescope of the disclosed concept. For example and without limitation, thewindings may be single-layer or multi-layer. The primary and/or secondcoils 40,42 may each have single or multiple windings. In embodimentswith multiple windings, the windings may be connected in series or inparallel for added design flexibility. Furthermore, each winding may beany of a variety of conductor styles such as, for example and withoutlimitation, copper foils, LITZ wire, or any other suitable conductorstyle. It will also be appreciated that the windings may be singlestrand or multi-strand without departing from the scope of the disclosedconcept. In example embodiments of the disclosed concept, shunt material50 may be disposed in selected portions of the space between the primaryand secondary coils 40,42. The shunt material 50 may be, for example andwithout limitation, a magnetic shunt material such as, for example andwithout limitation, ferrite material or powdered iron alloy. In someexample embodiments, the shunt material 50 may be a plate of material.The selected portions where shunt material is disposed may be selectedto tune characteristics of the multi-leg transformer 10 such as leakageinductance. That is, the shunt material 50 may be disposed in more orless of the volume between the primary and secondary coils 40,42 to tunethe leakage inductance of the multi-leg transformer 10 to a desiredlevel. Tuning of the leakage inductance is useful in transformerapplication such as, for example and without limitation, transformersused in LLC converters. Additionally, by being able to tune the leakageinductance via the shunt material 50, a separate resonant inductor tocontrol the leakage inductance element within the LLC circuit is notneeded.

In some example embodiments, the multi-leg transformer 10 has a lowprofile with, for example, short core legs and flat windings. The lowprofile enables better cooling by, for example, providing greater coresurface area for better cold plate cooling. Improved cooling is usefulin higher power applications. In some example embodiments, the multi-legtransformer 10 may be employed in, for example and without limitation,1-22 kW, and in some example embodiment in 4-7 kW applications. In someexample embodiments, the multi-leg transformer 10 may be employed in aLLC converter, but it will be appreciated that the multi-leg transformer10 may be employed in other applications such as, without limitation,battery charging applications, as a distribution transformer steppingdown voltages in large scale server applications, or in otherapplications.

In some example embodiments, the gap between core legs is minimal sothat any fringing flux is limited to a smaller area and allows for onlylimited interaction with the windings of the primary and secondary coils40,42.

In some example embodiments, the windings of the primary and secondarycoils 40,42 are flat windings of, for example and without limitation,solid or LITZ wire. In some example embodiments, flat windings minimizeproximity effect losses and allow for good coupling between the primaryand secondary coils 40,42. The windings may be made in series such thata winding around a core leg and an immediately adjacent winding around aneighboring core leg have current flowing in the same direction, as isshown by arrows denoting current flow in FIG. 7B. Current flowing thesame direction in immediately adjacent windings causes the fluxgenerated by the currents to cancel out. In example embodiments, tofacilitate immediately adjacent windings having current flowing in thesame direction, the transition from one winding to an immediatelyadjacent winding will have a wire going from an outside edge of onewinding to the inside edge of the adjacent winding, as is shown forexample in FIG. 7B. In some example embodiments, to facilitate isolationof winding transitions and prevent shorting under high voltageconditions, the bobbin may include features to route and isolate thewires in transition from one winding to another from each other.

In some example embodiments, the primary and secondary coils 40,42 arewound in a similar manner, but it will be appreciated that they may bewound differently. It will be appreciated that the primary and secondarycoils 40,42 may have the same or different numbers of turns. The primaryand secondary coils 40,42 may be configured in a matrix such that thewindings around each post may be connected in series or paralleldepending on voltage and current requirements. In some exampleembodiments, the primary and secondary coils 40,42 may be further beconfigured as tapped windings to allow for the use of lower voltagerated switching devices. In some example embodiments, the primary andsecondary coils 40,42 may or may not be molded to allow for better heattransfer and increased voltage isolation.

It will be appreciated that the bobbin may be formed from any number ofportions without departing from the scope of the disclosed concept. Forexample, while upper and lower bobbin portions 30,32 are shown in anexample embodiment, it will be appreciated that the upper and lowerbobbin portions 30,32 may be combined, or further split in various waysinto any number of bobbin portions. The upper and lower bobbin portions30,32 each include center portions corresponding to the center posts24,25,26. Each coil of the primary and secondary coils 40,42 are woundaround one of the center portions.

In some example embodiments, the upper and lower bobbin portions 30,32may include one or more capture features that may assist in keeping theprimary and secondary coils 40,42 flat and in an essentially spiralshape around each of the posts. An example of a capture feature 34 isshown in FIG. 6C. The capture feature 34 may be, for example and withoutlimitation, a shaped notch.

In some example embodiments, the upper and lower bobbin portions 30,32may further contain an insulating barrier between the coils in thecenter to increase the turn to turn isolation voltage of the outer mostturns in each coil.

In some example embodiments of the disclosed concept, the shunt material50 is varied by thickness, width, and/or length to tune the leakageinductance of the multi-leg transformer 10. The shunt material 50 may belocated in any or all of the spaces between the core center posts and/oroutside core legs.

While specific embodiments of the disclosed concept have been describedin detail, it will be appreciated by those skilled in the art thatvarious modifications and alternatives to those details could bedeveloped in light of the overall teachings of the disclosure.Accordingly, the particular arrangements disclosed are meant to beillustrative only and not limiting as to the scope of the disclosedconcept which is to be given the full breadth of the claims appended andany and all equivalents thereof.

What is claimed is:
 1. A multi-leg transformer comprising: a core havinga plurality of center posts; a primary coil wound around at least one ofthe center posts; a secondary coil wound around at least one of thecenter posts and spaced apart from the primary coil; and at least onemagnetic shunt material disposed in one or more selected areas betweenthe primary coil and the secondary coil.
 2. The multi-leg transformer ofclaim 1, wherein the at least one magnetic shunt material includes aplurality of pieces of magnetic shunt material.
 3. The multi-legtransformer of claim 2, wherein the plurality of pieces of shuntmaterial include different types of magnetic shunt materials.
 4. Themulti-leg transformer of claim 1, wherein at least one magnetic shuntmaterial is composed of at least one of ferrite material and iron alloy.5. The multi-leg transformer of claim 1, wherein one or more selectedareas are selected to tune leakage inductance of the multi-legtransformer.
 6. The multi-leg transformer of claim 1, wherein the coreis a multi-part core.
 7. The multi-leg transformer of claim 6, whereinthe multi-part core includes an upper core and a lower core, wherein oneof the primary coil and the secondary coil is wound around center postsof the upper core, and wherein the other of the primary coil and thesecondary coil is wound around center posts of the lower core.
 8. Themulti-leg transformer of claim 1, further comprising: a first bobbinstructured to facilitate winding and placing of the primary coil; and asecond bobbin structured to facilitate winding and placing of thesecondary coil.
 9. The multi-leg transformer of claim 8, wherein atleast one of the upper bobbin and the lower bobbin includes at least onecapture feature structured to assist the primary coil or secondary coilretain shape around the center posts. The multi-leg transformer of claim8, wherein at least one of the upper bobbin and the lower bobbin includean insulating barrier structured to provide insulation between coils ofthe primary coil or the secondary coil.
 11. The multi-leg transformer ofclaim 1, wherein at least one of the primary coil and the secondary coilhave flat windings.
 12. The multi-leg transformer of claim 1, wherein atleast one of the primary coil and secondary coil have windingsstructured such that current flowing through a first winding around afirst center post and current flowing through an immediately adjacentwinding around an adjacent second center post are in the same direction.13. The multi-leg transformer of claim 1, wherein at least one of theprimary coil and the secondary coil have tapped windings.
 14. Themulti-leg transformer of claim 1, wherein at least one of the primarycoil and the secondary coil have windings of solid or LITZ wire. Amethod of making a multi-leg transformer, the method comprising:providing a core having one or more core legs, a primary coil woundaround at least one of the core legs, and a secondary coil wound aroundat least one of the core legs and spaced apart from the primary coil;determining a desired level of leakage inductance of the multi-legtransformer; and tuning the multi-leg transformer to the desired levelof leakage inductance by placing at least one magnetic shunt material inone or more selected areas between the primary coil and the secondarycoil.
 16. The method of claim 15, wherein the at least one magneticshunt material includes a plurality of pieces of magnetic shuntmaterial.
 17. The method of claim 16, wherein the plurality of pieces ofshunt material include different types of magnetic shunt materials. 18.The method of claim 15, wherein the at least one magnetic shunt materialis composed of at least one of ferrite material and iron alloy.
 19. Themethod of claim 15, wherein tuning the multi-leg transformer includesselecting one or more of the location, volume, and type of the at leastone magnetic shunt material.
 20. A bobbin for a multi-leg transformerhaving a core having a plurality of center posts, a primary coil woundaround at least one of the center posts, a secondary coil wound aroundat least one of the center posts and spaced apart from the primary coil,and at least one magnetic shunt material disposed in one or moreselected areas between the primary coil and the secondary coil, thebobbin comprising: a plurality of center portions corresponding to theplurality of center posts, each center portion structured to have a coilof the primary coil or secondary coil wound around it; at least oneinsulating barrier disposed between two of the plurality of center postsand structured to provide insulation between coils of the primary coilor the secondary coil; and at least one capture feature structured toassist the primary coil or secondary coil retain shape around the centerposts.